It showed a complex, abstract, flowing form (perhaps from Trapcode Particular) superimposed on a video shot while walking down a sidewalk. The particular effect was move-matched, so that as the camera's POV changed from looking frontwards to sideways to looking back, the particles were integrated into the scene. It was as if the particles were walking down the sidewalk, and the camera overtook them.

Or so I remember - could be wrong.

At any rate it was incredible, and I'd like to learn how it's done. If you've seen that or a similar tutorial, please share!

Precisely the one. Now to persuade the creator to discuss how he did it. Have you explored these kinds of techniques? I really like the idea of tracking the real world along with particles. It transforms them from "cool" to "magic."

It might not take as long as you think. The video's creator writes the following in the comments section:

"Hi.
Thanks for all the encouragement. I certainly was not expecting such an amazingly huge (and FAST) response.

I added info up top to address the frequent questions about tracking. Pixel Farm's software is quite impressive, and can get some great results without much sweat. There was still trial and error to get some shots working, even tracking it in several pieces and joining the moves after. I'm still new to that sort of tracking software though.

I'll certainly consider making some materials to explain things a little."